Abstract: Stress Determination along the Andes Cordillera for Successful Horizontal Well Planning in the Llanos Basin, Colombia

Last, Nigel C. - BP Exploration Colombia; Marvin E. Markley - Schlumberger/ Geoquest

This study concentrated on the impact of rock mechanical and hole stability considerations on the planning of the first horizontal well in the foreland area of the Andes foothills in Colombia (Fig. 1).

The key challenge which would impact well design was the assessment of the stress state in a setting which was thought to be neither representative of the active tectonics in the nearby foothills, nor of a classical passive margin (Fig. 2). A methodology was developed to estimate the actual stress state in the reservoir and its overlaying sediments. Stress orientations were determined from borehole breakouts in offset vertical wells. An initial estimate of stress magnitudes was made by imposing the assumed constraint (based on structural interpretation) of a limiting strike-slip regime on the region. A sensitivity analysis was used to refine and confirm the stress state based on comparing the predicted critical mud weights from a range of other plausible stress models to the drilling performance and logging data from offset wells. The stresses associated with the model most consistent with the existing field data provided the basis for well trajectory planning and borehole stability evaluation in both the pilot and horizontal well. In addition:

- Drilling performance and wellbore stability studies from the nearby foothills gave valuable information and insight.

- Offset vertical wells in the field provided clear warning signs (cavings, high torque, high overpulls and stuck pipe) of potential difficulties in directional wells.

- Careful selection of casing setting depths could alleviate some of the instability.

- Mud weight ranges for stable hole were calculated from the deduced stresses.

- Natural directional ?drift? tendencies of non-steered off -set vertical wells indicated strong correlation with structural dip and therefore the least problematic drilling direction.

The planning process lead to the following important pre-drill recommendations for implementation of the proposed horizontal well:

- The well should be drilled from North to South (close to S10W) to capitalize on the anticipated natural drift tendency (up-dip), reduce directional work and minimize hole instability problems (parallel to the minimum horizontal stress).

- The calculated mud weight ranges should be used to guide mud weight selection to mitigate against hole instability and associated drilling problems.

- All reasonable measures should be taken to monitor open hole condition (for example, excessive cavings), particularly in the build section. Be prepared to react.

- A pilot hole should be drilled to confirm the planning and design assumptions, and additional selective data acquisition (for example logging to obtain formation tops and dips) used to optimize the drilling of the subsequent build section.

The well was drilled successfully to plan, demonstrating the value of understanding the stress state and implied design issues when planning the first well of this type in a new and somewhat unusual geological setting. Indeed there had been initial conjecture that the well could not be drilled. The multidisciplinary, integrated approach was a key factor in determining the successful outcome.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil